Why is blender treating vertices differently when applying subdivision surface modifier?
To keep it simple:

If the faces normals are in the same orientation, they can be smoothly interpolated at the subdivision: their mean stays positive.



If they are not, their mean becomes null and so their is no interpolation possible and their mean (so the orientation) is null.



This is like to say the function is not derivable at the face junction.

So the junction stays a singularity and stays angular.